1. Field of the Invention
This invention relates to surgical instrumentation and processes for the anterolateral surgical correction of such conditions as scoliosis, which is also known as curvature of the spine.
2. Description of the Related Technology
Thoracolumbar pathologies such as spinal fractures, spinal tumors, kyphosis and scoliosis in humans may occur as a result of many different causes. Scoliosis for example may occur as a result of a disease such as polio, paralytic diseases of neuromuscular etiology, or injury to the spinal column. However, the most common cause of scoliosis in first world countries is a genetically determined growth abnormality of the spinal column which most often characteristically causes the curve to develop when the children are passing from late childhood through adolescence. This condition is known as idiopathic scoliosis.
While prevention and bracing can be effective for some children who develop scoliosis, surgical treatment is commonly when employed when the spinal curvature is too pronounced to respond to bracing or when established curves threaten a normal productive, pain free adult life. The standard surgical treatment for scoliosis since the mid-1950's has been an “instrumented spinal fusion,” which typically involved the implantation of metal articles such as hooks or screws to the spinal column at each end of the curve. Retaining rods are then attached to the hooks or screws at the ends of the curve. Surgical instruments are then mechanically used to straighten the spinal column (by twisting the spinal column or jacking it up) and the rods are then attached to the hooks or screws and fixed into place to maintain the position of the spinal column in the lengthened, straightened and corrected position. Surgery may be performed using the anterolateral approach, in which correction of the vertebrae is performed from the patient's front or side or the posterior correction method in which correction of the vertebrae is performed from the rear.
To prevent subsequent loosening of the implants and loss of correction of the deformity, a spinal fusion of the instrumented section of the spinal column is virtually always performed at the same time as the instrumentation. This means that bone chips are placed along portions of the spinal column not covered by the implants. These bone chips or grafts induce the vertebrae which were part of the curvature to grow together (fuse) over a period of weeks to months to years. This fusion maintains the correction of the spinal deformity achieved by the application of the instruments (implants). For many years, the predominant surgical approaches to spinal instrumentation tended to correct the curvature incompletely, and typically instrumented and fused long segments of the spinal column, most usually 7-14 segments. Such an extensive procedure was unavoidably traumatic to the patient and requires a great deal of recovery time, sometimes more than a year. Those approaches also left behind spinal implants which, because of their size and bulk, commonly cause problems after their implantation. The profile of these implants, which can be defined as their distance of extension beyond the normal vertebral structure of the patient's spine, can interfere with the muscle in the lumbar spine such as the iliopsoas muscle, the nerves of the lumbar plexus and other critical anatomical structure such as ribs, blood vessels, lungs, the liver and the heart. One such approach is depicted in FIG. 1. This approach, which is described in great detail in U.S. Pat. No. 5,603,714, includes a system 10 for fusing a number of vertebral bodies 12 that utilizes a number of staple elements 14 that have tines for penetrating the vertebral body. As may be seen in FIG. 1, each staple element 14 is anchored to a respective vertebral body 12 by a pair of vertebral screws 16, which extend through apertures 18 defined in the staple elements 14 and each of which includes a threaded portion 20 for penetrating the vertebral body and a head portion 22. Each head portion 22 has a channel 24 defined therein for receiving a retaining rod 26. Each head portion 22 further includes a set screw 28 for finally securing the vertebral screw 16 to the retaining rod 26 at the conclusion of the surgical procedure. As may be seen in FIG. 1, the head portions 22 of the vertebral screws 16 extend significantly beyond the circumferential outer surfaces of the vertebral bodies 12. Accordingly, the system may be said to have a relatively high profile.
Significant improvements in both surgical technique and instrumentation for the treatment of scoliosis were first disclosed in U.S. Pat. No. 6,524,311 to Gaines, Jr. The Gaines patent introduced a “bone on bone” surgical technique in which bone to bone apposition between the vertebrae was disclosed as possible, and, in fact, as a goal. No previous approach to surgical correction ever utilized complete discectomy as a part of the surgical technique to achieve bone-on-bone apposition through the fusion area. This technique both eliminated structural barriers to full correction and permitted the quality of intimate apposition of the vertebrae in the curvature, which facilitates rapid healing (2-3 months) of the operated fusion. The Gaines patent also disclosed advantageous low-profile instrumentation for use with such a surgical process.
The bone on bone surgical technique has proved to have been a remarkable success in reducing the amount of time that is required to recover from spinal recovery surgery. However, as in the case of all spinal corrective surgery, a significant amount of instrumentation is still required to perform the bone on bone technique. This instrumentation will remain permanently attached to the patient's spinal column throughout the patient's lifetime. This poses potential risks to the patient's long term health.
One conventional method of surgical correction of certain thoracolumbar pathologies such as spinal fractures and spinal tumors involves the complete removal of a vertebral body and the reconstruction of the spine in the affected area by the implantation of a spinal cage that is interposed between the vertebral bodies that are immediately above and beneath the section that has been removed. The cages can be made of metal, carbon fiber, or allograft bone. Bone material may be packed into the spinal cage to accelerate healing of the affected area. In such procedures, metallic bone screws are typically implanted into the healthy vertebrae in order to constrain the spinal cage against movement during the postoperative healing process. These bone screws will remain permanently attached to the patient's spinal column throughout the patient's lifetime.
The term “bioabsorbable” as it is used herein is interchangeable with “bioresorption” refers to a material or materials that degrade as a result of cellular activity (e.g., phagocytosis) in a biological environment. As used herein in reference to polymers, the term “degrade” refers to cleavage of the polymer chain, such that the molecular weight stays approximately constant at the oligomer level and particles of polymer remain following degradation. The term “completely degrade” refers to cleavage of the polymer at the molecular level such that there is essentially complete mass loss. The term “degrade” as used herein includes “completely degrade” unless otherwise indicated.
Bioabsorbable materials have been used for surgical and orthopedic purposes, but never for anterior correction of thoracolumbar spinal pathologies such as scoliosis because the forces between the vertebrae in the sacral, thoracic and lumbar spine have in the past generally been considered to be too large for the safe application of such materials. U.S. Pat. No. 6,719,795 to Cornwall et al. discloses a resorbable posterior spinal fusion system for use in procedures in which instrumentation is secured to the posterior or rearward side of the spine. However, the forces and stresses that are imparted to posterior instrumentation and the screws that secure it to the vertebrae are always much less than what would be imparted to anterior instrumentation secured to the same vertebral body. Cornwall et al. provide no guidance for those who seek to improve upon existing therapies for anterior correction of thoracolumbar spinal pathologies such as scoliosis.
A need exists for an improved system and method for performing corrective surgery for spinal conditions such as scoliosis that is less traumatic to and facilitates a more rapid recovery for the patient, and that utilizes implants that present fewer short and long term postsurgical problems to the surrounding anatomy of the patient.